1. Field of the Invention
This invention relates generally to seismic surveying, and, more particularly, to adding a signal to seismic data.
2. Description of the Related Art
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In a typical marine seismic survey, an array of marine seismic streamer cables is towed behind a survey vessel over a survey area. The seismic streamer cables may be several thousand meters long and contain a large number of sensors, such as hydrophones and associated electronic equipment, which are distributed along the length of the each seismic streamer cable. The survey vessel also tows one or more seismic sources, such as airguns and the like.
As the array is towed over the survey area, acoustic signals, or “shots,” produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various subterranean geological formations. The reflected signals are received by the sensors in the seismic streamer cables, digitized and then transmitted to the survey vessel. The digitized signals are referred to as “traces” and are recorded, and at least partially processed, at the survey vessel. The ultimate aim of this process is to build up a representation of the subterranean geological formations beneath the array. Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations.
In addition to detecting the acoustic signals, the sensors in the seismic streamer cables may also detect acoustic noise from a variety of sources. For example, hydrophones distributed along the seismic cable may detect swell noise. The sensors do not distinguish between the desired acoustic signals and the undesirable acoustic noise, and so the recorded traces include both signal and noise. The accuracy and commercial value of the representation of the subterranean geological formations depends, at least in part, on the signal-to-noise ratio of the seismic data. For example, increasing the signal-to-noise ratio of the traces from a seismic survey typically allows a more accurate, and consequently more commercially valuable, representation of the subterranean geological formations to be formed. However, estimates of the signal-to-noise ratio of seismic data are difficult to obtain because the recorded acoustic signals are virtually always accompanied by an unknown amount of acoustic noise.